2.1. ALZHEIMER'S DISEASE
Sporadic Alzheimer's Disease is the major neurodegenerative disease associated with aging, the risk of developing the disease rising exponentially between the ages of 65 and 85, doubling every five years. Histologically, the hallmarks of Alzheimer's Disease are the deposition of amyloid in senile plaques and in the walls of cerebral blood vessels; the presence of neurofibrillary tangles, and neurodegeneration. The etiology of Alzheimer's Disease, however, is not well understood. Genetic factors have been proposed to play a role, including trisomy 21, mutations in the amyloid .beta.-protein precursor ("APP") gene, the presenilin-1 (PS1) and presenilin-2 (PS2) genes, and the presence of the apolipoprotein E type 4 allele (Younkin, 1995, Ann. Neurol. 37:287-288; Lendon et al., 1997, JAM A 277:825). Several studies have indicated that .beta.-amyloid induces apoptosis in cultured neurons (Loo et al., 1993, Proc. Natl. Acad. Sci. U.S.A. 90:7951-7955; Li et al., 1996, Brain Res. 738:196-204). Such induction may involve the immediate early gene proteins, c-jun and fos (Anderson et al., 1995, J. Neurochemistry 65:1487-1498; Anderson et al., 1994, Experimental Neurology 125:286-295; Anderson et al., 1996, J. Neurosci. 16:1710-1719).
It has been proposed that apoptosis may be involved in the pathogenesis of Alzheimer's disease Smale et al., 1995, Exp. Ncurol. 133:225-230; Anderson et al., 1996, J. Neurosci. 16:1710-1719; Anderson et al., 1994, Exp. Neurol. 125:286-295. Inflammatory mechanisms have also been implicated; (Pasinetti, 1996, Neurobiol. Ageing 17:707-716) supportive of such a mechanism are the observations that acute phase proteins are elevated in the serum of Alzheimer's Disease patients, and are deposited in amyloid plaques; activated microglial cells tend to localize in the vicinity of senile plaques, and complement components have been localized around dystrophic neuntes and neurofibrillary tangles (Aisen and Davis, 1994, Am.J.Psychiatry 151:1105-1113). Antuinflammatory agents have been suggested as potential therapeutic agents (Aisen et al., 1996, Dementia 7:201-206; McGeer et al., 1996, Neurology 47: 425-432). Because they tend to have fewer adverse side effects, selective inhibitors of the enzyme cyclooxygenase-2 have been advanced as agents for treating such inflammation (International Publication No. WO 94/13635 by Merck Frosst Canada Inc.). Prior to the present invention, however, it had not been believed that such agents could be used to inhibit non-inflammatory aspects of neurodegeneration in the context of Alzheimer's Disease or otherwise.
2.2. CYCLOOXYGENASE-2
Cyclooxygenases ("COXs") are enzymes that catalyze the formation of prostaglandin ("PG")-H.sub.2 from arachidonic acid (AA). PG-H.sub.2 is further metabolized to physiologically active PGs (e.g., PG-D.sub.2, PG-E.sub.2 and PG-F.sub.2.alpha.), prostacyclin (PG-I.sub.2) and thromboxanes. Specific PGs have diverse, often antagonistic effects on different tissues. For example, PG-I.sub.2 and PG-E.sub.2 are potent vasodilators that may contribute to the inflammatory response, whereas PG-F.sub.2.alpha. is a vasoconstrictor.
There are two known COX isoforms, COX-1 and COX-2, which, though physiologically distinct, are similar in amino acid sequence and enzymatic functions. COX-1 is constitutively expressed at different levels in different cell types. COX-2, however, is not constitutively expressed, and is generally undetectable in normal peripheral tissues (Kujubu et al., 1991, J. Biol. Chem. 266:12866-12872; O'Banion et al. 1992, Proc. Natl. Acad. Sci. U.S.A. 89:4888-4892). Rather, COX-2 expression is inducible (for example, by mitogens) and COX-2 mRNA levels have been observed to rise rapidly in response to inflammatory stimuli such as interleukin-1.beta. and lipopolysaccharide, and to decrease in response to glucocorticoids. When subjected to these same factors, COX-1 mRNA levels remain substantially unchanged, suggesting that COX-2 is the isoform which mediates inflammation (Cao et al., 1995, Brain Res. 697:187-196; O'Banion et al. 1992, Proc. Natl. Acad. Sci. U.S.A. 89:4888-4892).
Recent evidence suggests that COX-2 may play a role in mechanisms of cell survival and cell adhesion in peripheral cells (Lu et al., 1996, Proc. Natl Acad. Sci. U.S.A. 92:7961-7965; Tsujii et al., 1995, Cell 83:493-501). Tsuji et al. reports that epithelial cells engineered to express elevated levels of COX-2 were resistant to butyrate-induced apoptosis, exhibited elevated BCL2 protein expression, and reduced transforming growth factor .beta.2 receptor levels (Tsuji et al., 1995, Cell 83:493-501). Lu et al. indicates that non-steroidal antiinflammatory drugs may induce an apoptotic mechanism involving the COX system.
The roles of COX-1, COX-2 and PG synthesis in normal brain, and in the context of Alzheimer's Disease, have not been fully characterized to date. The importance of PGs in brain physiology may be independent of inflammatory mechanisms. In the brain, PG receptors have been identified in the hypothalamus, thalamus, and limbic system (Watanabe et al., 1989, Brain Res. 478:143-148). PGs are involved in hypothalamic-pituitary hormone secretion (Kinoshita et al., 1982, Endocrinol. 110:2207-2209), regulation of temperature and the sleep-wake cycle (Hayaishi, 1988, J. Biol. Chem. 263:14593-14596). There is recent evidence that COX-2 mRNA is expressed and regulated in rat brain by synaptic activity and glucocorticoids (Adams et al., 1996, J. Neurochem. 66:6-13; Kaufmann et al., Proc. Natl. Acad. Sci. U.S.A. 93:2317-2321; Yamagata et al., 1993, Neuron 11:371-386). These studies indicate that COX-2 is regulated as an immediate early gene in the brain, and suggest that PGs may be important in trans-synaptic signalling and long-term potentiation. Chang et al. (1996, Neurobiol. of Aging 17:801-808) report that COX-2 mRNA expression is decreased in Alzheimer's disease.